During the fabrication of integrated circuits such as memory devices, it is conventional to test such integrated circuits at several stages during the fabrication process. For example, the integrated circuits are normally connected to a tester with a probe card when the integrated circuits are still in wafer form. In a final test occurring after the integrated circuits have been diced from the wafer and packaged, the integrated circuits are placed into sockets on a load board. The load board is then placed on a test head, typically by a robotic handler. The test head makes electrical contact with conductors on the load board that are connected to the integrated circuits. The test head is connected through a cable to a high-speed tester so that the tester can apply signals to and receive signals from the integrated circuits.
During the testing of integrated circuits, it is often necessary to supply a periodic signal, such as a clock signal, to the integrated circuit. The frequency of the periodic signal is determined by the nature of the integrated circuit. It is sometimes necessary to apply a periodic signal to an integrated circuit having a frequency that varies over a wide range. For example, during speed grading of integrated circuits, such as memory devices, it is necessary to increase the frequency of a clock signal from an initial frequency to a frequency that is high enough for the integrated circuit to no longer function properly. Additionally, it is often desirable to supply several different phases of a periodic signal, such as an in-phase periodic signal and a quadrature periodic signal. For example, it may be necessary to apply data signals to a memory device in synchronism with an in-phase periodic signal, and to apply a write strobe signal to the memory device at the quadrature of the in-phase signal. It is also sometimes necessary in applications other than testing to provide periodic signals at one or more phases that vary over a wide range of frequencies.
A variety of techniques have been used to generate periodic signals having a variable frequency. Voltage controlled oscillators (“VCOs”) using a variety of designs both analog and digital are frequently used. The frequency of a periodic signal generated by a VCO can be varied by simply varying the magnitude of a control voltage applied to a control input of a VCO. Conventional VCO designs can also generate periodic signals at several different phases.
Another approach to generating a periodic signal having a variable frequency is to use a phase-lock loop (“PLL”). A phase-lock loop also uses a VCO, but the phase of the signal generated by the VCO is compared with the phase of a reference signal using a phase detector. The phase detector generates an error signal having a magnitude corresponding to a difference in phase between the two signals, and this error signal is used to adjust the frequency (i.e., change in phase with time) of the signal generated by the VCO. The periodic signal generated by the VCO and the reference signal will, of course, have the same frequency if their phases are kept equal to each other. The frequency of the signal generated by the VCO can be varied by coupling the periodic signal from the VCO to the phase detector through a divider circuit that divides the frequency of the periodic signal by a variable number N. The periodic signal generated by the VCO will then have a frequency of N*f0, where f0 is the frequency of the reference signal.
The use of a VCO, either alone or as part of a PLL, has the advantage of being relatively simple and using a relatively little amount of circuitry. However, conventional VCO's are generally able to operate over only a limited range of frequencies. Many applications require that the frequency of a periodic signal be varied over a range that is wider than the range of frequencies that can be achieved using a VCO.
Another approach is to use a bank of VCOs, each of which operates over a limited range of frequencies. While this approach addresses the shortcomings of using a single VCO, it results in a relatively complex and expensive solution to the limitations inherent in using a single VCO.
There is therefore a need for a relatively simple and inexpensive system and method that is able to generate a periodic signal over a wide range of frequencies, and preferably at several different phases.